The invention relates to cathodic arc plasma sources, and more particularly to method and apparatus for the initiation of the cathodic arc.
Cathodic arc plasma deposition is a coating technology with great potential. Most importantly, cathodic arc plasmas are fully ionized and can therefore be manipulated with electric and magnetic fields. While electric fields are used to change the ion energy and thus the structure and properties of deposited films, magnetic fields are used to guide and homogenize the plasma.
However, a major obstacle to the broad application of cathodic arc plasma coating is the presence of macroparticles in the plasma, where macroparticles broadly encompasses all particles much larger than the ions, including droplets, microparticles, and nanoparticles.
Cathodic arc current is localized in minute nonstationary cathode spots. Spot formation is necessary to provide sufficient power density for plasma formation, electron emission, and current transport between the cathode and anode. Macroparticles originate from plasma-solid interaction at cathode spots.
Many approaches have been proposed and tested to eliminate macroparticles from cathodic vacuum arc plasmas. Most successful are curved magnetic filters, originally introduced by Aksenov and co-workers in the late 1970s. Although high-quality metal, metal-compound, and diamond-like carbon films have been synthesized by filtered cathodic arc deposition, macroparticle filters suffer from two major drawbacks: (1) the plasma transport is inefficient, i.e. only a fraction of the original (unfiltered) plasma is actually useable for film deposition, and (2) the removal of macroparticles is not complete. The latter is particularly pronounced for solid macroparticles as observed with cathodic arc carbon plasmas.
The design of macroparticle filters depends first and foremost on the mode of arc operation. DC arc plasma sources are usually equipped with cathodes of large size, e.g. diameter of 3-5 cm. The spot location may be magnetically controlled. In any case, the location(s) of plasma production, the micron-size cathode spot(s), can vary across the cathode surface, and the cross section of the filter entrance must be large enough to accommodate the various spot locations. A large filter entrance necessarily implies a large filter in length, volume, and weight. However, the plasma density in the filter drops exponentially with the path length of the filter.
Most filters, and virtually all of the DC-operated filters, have a xe2x80x9cclosedxe2x80x9d architecture in the sense that the filter volume is enclosed by a tube or duct which is surrounded by magnetic field coils. Macroparticles cannot leave the filter volume. They are expected to stick to the duct wall or to be caught between baffles that are placed inside the duct. The ducts are preferably bent, e.g. at 45xc2x0 or 90xc2x0, so there is no line-of-sight from the arc spot to the substrate.
U.S. Pat. No. 6,031,239 shows a filtered cathodic arc source with a filter of closed architecture having a toroidal duct with two bends, preferably in different planes, and a liner or baffle in the duct. The double bend provides no line-of-sight and no single bounce path through the duct. The duct is relatively large, with a diameter of 4-6 inches.
However, catching macroparticles is difficult for some cathode materials such as carbon because the macroparticles tend to be elastically reflected from surfaces. This xe2x80x9cbouncingxe2x80x9d problem is addressed by filters with open architecture where xe2x80x9cbouncingxe2x80x9d is used to let macroparticles escape from the region of plasma transport. Filters of open architecture do not have a duct but consist of a few turns of a magnetic field coil. The coil must have a relatively high current to generate sufficient field strength despite the small number of turns per length. For convenience, the arc current can be used in the filter coil.
Thus a short, open-architecture magnetic filter in combination with a compact arc source with a cathode of small area and operated in pulsed mode is desirable in order to have a high throughput of clean plasma to a deposition target.
Accordingly it is an object of the invention to provide an improved method and apparatus for arc initiation in a cathodic arc plasma source.
The invention is a xe2x80x9ctriggerlessxe2x80x9d arc initiation method and apparatus based on simply switching the arc supply voltage to the electrodes (anode and cathode). Neither a mechanical trigger electrode nor a high voltage flashover from a trigger electrode is required. A conducting path between the anode and cathode is provided, which allows a hot spot to form at a location where the path connects to the cathode. The resistance of the path is preferably in the 1xcexa9-10 kxcexa9 range. While the conductive path is eroded by the cathode spot action, plasma deposition ensures the ongoing repair of the conducting path.
Arc initiation is achieved by simply applying the relatively low voltage of the arc power supply, e.g. 500 V-1 kV, with the insulator between the anode and cathode coated with a conducting layer and the current at the layer-cathode interface concentrated at one or a few contact points. The local power density at these contact points is sufficient for plasma production and thus arc initiation.
A conductive surface layer, such as graphite or the material being deposited, is formed on the surface of the insulator which separates the cathode from the anode. The mechanism of plasma production (and arc initiation) is based on explosive destruction of the layer-cathode interface caused by joule heating. The current flow between the thin insulator coating and cathode occurs at only a few contact points so the current density is high.
A cathodic arc plasma deposition system using a source with this triggerless arc initiation system can be used for the deposition of ultrathin amorphous hard carbon (a-C) films for the magnetic storage industry.